This invention relates to the forming of a substantially solid skin on the surface of a foam material. More particularly, it relates to the method by which the skin is developed during the formation of the foam. In the past, skin foam materials have been made for a number of reasons. A skin gives the foam a superior outer appearance in that the foam structure has a look of substance. Similarly, the skinned foam is resistant to surface abrasion and cutting when used, for example, as disposable mealservice. Skinned foam acts as a stiffened structure in the same way as an I-beam or a honeycomb sandwich when bent in that the surface skin is removed relative to the central plane of the structure, thus enabling a lighter and/or thinner overall structure having a maximum bending stiffness. Greater bending stiffness is achieved since the modulus of the foam tends to decrease more rapidly than the foam density, and the skin is carried on the surface or at the area where stress is maximum.
The prior patent art is full with techniques for forming skinned foam structures. The Haas U.S. Pat. No. 3,925,526 patent incorporates heat sinks in the mold in order to obtain areas of greater density on injection molded products. Selectively cooling different parts of the mold condenses and redissolves gasses in the foam to give a thicker skin on the injection molded parts. The feed for the injection molded parts is homogeneous and there is no stratification by the various temperatures of the mold components.
National Rubber Machine Corporation, of Talmadge, Ohio, has developed an extrusion process for skinned foam. Different melt streams are combined in a co-extrusion die, one of which streams contains foaming agent. In order to assure non-mixing flow through the die so that the separate foaming and nonfoaming layers remain distinct, a low viscosity resin is used for the layers intended to be the foam skin. This is done to match the viscosity of the solid or skin layers to the foaming agent containing layers. This process has a number of disadvantages. Low viscosity resin tends to be more brittle and less heat resistant than the higher viscosity resin required for foaming. In addition, the scrap generated by such a process contains a mixture of low and high viscosity material. This scrap, which is typically between one-third and one-half of the extruded sheet in an extrusion-thermoforming operation, is difficult to use in the foam layer because the low viscosity fraction is difficult to foam, and is difficult to use in the solid or skin layer because the high molecular weight resin contamination raises the viscosity of this layer, where low viscosity is desired.
The Fillmann U.S. Pat. No. 3,972,664 shows thermoplastic multilayer bodies wherein the layers are of different densities. The injection molding process used includes different melt streams; one of which has a foaming agent and the other has no foaming agent. The streams are introduced to the mold sequentially thus the core and skin are produced independently. Such a system is slow and difficult to perform in that two steps must be sequentially executed and since the processing conditions for each step vary widely, readjustment time delays for setting the mold temperature may be necessary.
A melt flow separator in a coextrusion process for applying thermoplastic insulating compounds to a wire is shown in the Swiatovy, Jr., U.S. Pat. No. 4,093,414. Here different density polymers are extruded through a single die. The resulting surface on the foam is smooth but there is no control of the particular skin configuration i.e., the density or thickness of the skin relative to the foam. The Oisugu U.S. Pat. No. 3,713,762 has a breaker plate to divide the extruded plastic into two flows, one foaming and the other an unfoamed exterior cover layer. The separated flows are kept at different temperatures such that the skin does not foam. The Aishima et al U.S. Pat. No. 3,857,914 and Cherney U.S. Pat. No. 3,311,681 disclose other approaches to surface chilling for producing skin foam. The Nitta et al U.S. Pat. No. 3,843,285 shows rotational molding of resin powders that are centrifugally forced to a wall of the mold and which become skinned by introduction of steam on the rotated mold surface. Finally, Wade U.S. Pat. No. 3,470,055 shows an extrusion coating process for providing a polymer surface laminated to a subsurface by cooling under a chilled roll.
The prior art techniques disclose processes that are limited since they may not have the advantages and economics of a single die that permit good control of the difference in strength, density and thickness of the skin relative to the foam core or require use of materials which introduce problems in subsequent processing steps. Similarly, techniques using multiple dies or lamination of materials to produce the difference in density, strength, material and/or thickness for the core relative to the surface are complicated and uneconomical.